1. Field of the Invention
The present invention relates to dental and orthopedic implants including, for example, artificial tooth, artificial dental root, artificial bone, artificial articular, bone filler, bone screw, bond plate and bone frame. The present invention relates also to a method of making such implants.
More particularly, the present invention relates to an improved implant which is excellent in the affinity to bone tissue, and a method making of the same. The improved implant comprises a core and an anodic oxidation film formed on the core. The anodic oxidation film contains P and Ca in order to improve the affinity to the bone tissue in which the implant is implanted.
2. Related Background Art
In recent years, a remarkable progress has been made in medical techniques. With the arrival of "graying society", people are expecting a further and rapid development of this technical art. One developing technical field relates to new materials for bone substitute and bone reinforcement such as artificial dental root, artificial bone and artificial articular. They are now being accepted in the medical art more widely and rapidly. These new materials are generally called "implant" or "implant material". Mostly, they are made from metal, ceramics and other similar material.
Examples of the metallic material used for those implants which have already been put to practical use include stainless steel, Ni-Cr alloy, Co-Cr alloy, titanium and alloy thereof, and noble metal and alloy thereof. The selection of the most suitable material depends on the application of the final implant. Among others, at present, titanium and alloy thereof are preferably used since they have favorable properties such as high corrosion resistance, good bio-compatibility and excellent mechanical characteristics.
For those implants to be used, in particular, as artificial dental root and artificial bone, it is desirable that after implantation, the implant should be surrounded by the bone tissue as much as possible so as to keep the implant effective for a long time and stable in the living body. In order to attain this object, many methods have been proposed. One of the potential methods is to improve the affinity between implant and bone tissue by additionally treating the surface of the implant. According to a method proposed for this purpose, the surface of a titanium base body is coated with a layer of a biologically active (or compatible) material such as hydroxyapatite or other calcium phosphate compound by using the known plasma-spray technique thereby enabling a direct bonding between the implant surface and the bone tissue. Another method is to form a rough surface on the implant by plasma-spraying titanium powder. In a similar method, a porous surface layer is formed on the implant by sintering on it a large number of beads made of titanium or titanium alloy. These methods aim at increasing the mechanical engagement between implant and bone tissue so as to obtain a sufficient retention for the implant. However, it has been found that, as a practice matter, satisfactory implant cannot be prepared by using the techniques now available in the art as described above.
Besides the above methods, there are now being developed various means for obtaining the sufficient retention of the implant not only through the chemical bonding strength but also through some mechanical engagement between an implant and bone. For example, it has been proposed to form a large number of pores on the implant base plate by machining or cutting on it a thread. It has also been attempted to provide the implant with a rough surface by a chemical treatment, for example, etching with acid. There is also an attempt to additionally provide the rough surface with a coating of a biologically active material. In this case, it is essential for the coating to be stable in the living body and not to be exfoliated by the attack of biological cells or by degradation.
In the present standard of technique, however, it is very difficult to uniformly and firmly coat an implant having any complicated shape with biologically active material.
For example, by the plasma-spray technique, it is relatively easy to coat the outer surface of an implant with biologically active material. However, the coating of the inner surfaces of small through-holes and cylindrical ring portions is very difficult because the powder can hardly reach there. Similarly, in the case of those porous implants which have titanium beads or titanium alloy beads sintered on the implants and also those porous titanium implants to be used for filling the lost portion of a bone, it is very difficult to coat the whole surface uniformly with such biologically active material. Another problem involved in the coating of implants is found in that the adhesion of the coating to the base body is not sufficient to keep the coated implant effective for a long time under the severe conditions in the living body. In addition, the coating requires the use of a special expensive apparatus. Furthermore, the yield of the expensive hydroxyapatite is low and, therefore, no substantial coat-down is attainable.
In the art, besides the plasma-spray technique, there is also known a coating method in which a titanium base body of an implant is immersed in a solution containing Ca and P compounds and then the base body is baked so as to cover it with a layer of calcium phosphate. This method has an advantage that it does not impose so many limitations on the shape of the implant to be coated. However, an important drawback of the method is found in that the process of immersion coating--baking has to be repeated many cycles in order to form a film having a sufficient thickness for attaining the desired effect of the biological activity and that the process is complicated to carry out. In addition, the bio-active film obtained according to the method has an insufficient stability in the living body. In principle, by either of the above plasma-spray technique or the immersion--baking method, it is difficult to firmly coat the surface of a titanium base body with such ceramic material which is far different from the metal titanium. The two materials have a large difference in thermal expansion coefficient, crystal structure etc.
In the art there is also used the technique of anodic oxidation for attaining the above-mentioned purpose. In this method, a titanium anode and a cathode of, for example, stainless steel are placed in electrolyte solution and a voltage is applied between the two electrodes so that an oxidation film can be formed on the titanium anode as a result of the electrochemical oxidation of the titanium surface. Hitherto, this technique has been used, for example, for preparing colored titanium useful for accessories and architectural material. The colored titanium obtained according to this method is a kind of interference film the thickness of which is very small. The artificial dental root made of titanium can be colored in gold by employing the method. For this reason, this method has hitherto been used for color-matching of artificial dental root to gum. Compared with other known methods, this coating method has many advantages. Firstly, one can easily form a reactive oxidation film more than 1 .mu.m thick. Secondly, the adhesion between the oxidation film and the base body is good. Thirdly, any base body having a complicated shape can be coated uniformly. Finally, it needs no special apparatus and only a short time is required for carrying out the treatment.
Though the anodic oxidation method has many advantages as mentioned above, it has been found that the affinity between an implant and bone tissue is not always improved by this method so long as the oxidation film is composed of titanium oxide only.
Under the circumstances, in order to obtain such implants which satisfy all of the desired characteristics, it is absolutely necessary for the anodic oxidation method to be further improved. The present invention has been made to overcome the disadvantages of the prior art methods as mentioned above.